This invention relates to a process for removing solids from wastewater, waterwater sludge, and from the effluent of bacterial-algal treatment ponds.
When raw wastewater containing biodegradable organic contaminants is treated by oxygenation in contact with active biomass, a waste sludge is generated. As more municipalities and industries adopt this method for clean-up of sewage, the disposal of waste sludge becomes a larger problem.
Many waste streams are currently treated in algal ponds wherein algae supply oxygen and consume a portion of the waste stream's contaminants. However, as environmental laws become stricter, further removal of solids from the algal pond's effluent becomes necessary.
A common method of separating solids from liquids involves gravity settling. The difficulty with this method is that it requires long retention time for the material treated and correspondingly large equipment size and capital investment.
Dissolved air flotation is another method of separating solids from liquids. At least a portion of either the solids-containing stream or recycled effluent is pressurized with air, causing the air to dissolve therein. The pressurized stream is then expanded into a flotation vessel having lower pressure, causing the dissolved gas to come out of solution forming many small bubbles. Any unpressurized solids-containing liquid is also fed to the vessel. The air bubbles cling to or are occluded in the solid particles, causing said particles to float to the surface, where they may easily be removed by a conventional surface skimming device. The difficulty with this method is that compressing the air and pumping the liquid to be pressurized requires large expenditures of energy. Furthermore, recycling sufficient liquid for dissolving enough air to float the solids requires use of a large flotation vessel.